1. Field of the Invention
The process according to the invention relates very generally to the mass production of greens from a slurry comprising a divided solid phase, containing at least one metallic or ceramic powder placed in suspension in an aqueous or non-aqueous liquid phase.
The process consists of injecting the slurry into a mold cooled to a temperature lower than the solidification temperature of the slurry, then after solidification of the green thus obtained, of ejecting said green from the mold. Said green is then vacuum dried and then sintered at a temperature according to its composition in order to obtain the finished part.
2. Discussion of the Background
Various operations have shown that this process enables parts with a high density and very good characteristics (physical, mechanical, magnetic, electrical and so on) to be obtained after sintering.
Published patent applications WO 88/07902 and WO 88/07903 describe in a very detailed manner and by way of example the requirements for production of slurries containing metallic or non-metallic powders.
The first of these applications concerns the use, as a liquid phase, of water with different added compounds acting as cryoprotectors and as dispersants, and the second application uses non-aqueous compounds as a liquid phase.
The examples given in these two documents show that, with the exception of the case in which the suspension of the powder, or mixture of powders, is done in an organic liquid at a raised solidification temperature, molding of the slurry is carried out in a mold brought to a temperature significantly lower than 0.degree. C., so that the temperature difference between the mold and the solidification temperature of the slurry is sufficiently great to obtain very rapid solidification of the green with reduced risk of heterogenization of the solid-liquid mixture.
In the examples given in application WO 88/07902, a mould temperature of -78.degree. C. is used.
This working method enables very solid greens to be obtained which are easy to handle, which are subsequently dried by vacuum sublimation without risk of fissuring or deformation. During sintering very high actual densities on the order of 99% or more of the theoretical density can be achieved, something not known to be achievable using conventional methods of compression and sintering of powders, or by classical injection molding processes using thermoplastic binders.
Cooling of the molds is done in a known manner, for example by circulation of a coolant fluid cooled by known means.
It has been noted that even when the opening time of a mold cooled in this manner is reduced to a minimum for extracting the solidified green, ice deposits form on the joint planes of the mold as well as on the walls. These deposits prevent normal functioning of the joint planes, increase with each cycle of production of a green, and the thickness rapidly becomes too great for operations to be carried out. These deposits rapidly bring the functioning of the machine to a halt.
In order to remedy this inconvenience, it was proposed to house the entire installation inside a large gas-tight vessel, in which the dew point of the air or the gas contained would be lowered to below the temperature of the mold.
Such a vessel is difficult to produce as it has to contain not only the mold but also the means for opening and closing it, the means for injecting the slurry into the mold and also the means for ejecting and for then receiving and transferring the green to the outside of the vessel.
As indicated above, it is necessary to have a mold temperature as low as possible to obtain quality greens, that is to say in practice reaching a level equal to or below -20.degree. C. and preferably approximately in a range of between -20.degree. and -100.degree. C. Understandably it is extremely difficult to produce a large vessel, in which the gas contained has a dew point lower than the temperature of a mold within the indicated range. It is above all very difficult to conceive of a vessel of this type in which the dew point of the gas contained is maintained at such a value during mass production, in a rapid cycle, of greens injected in this manner.
The possibility of avoiding ice formation on the wall of the mold has been researched, particularly at each opening thereof, without having to use large gas-tight vessels, and without using complicated devices for holding the greens and transferring them to the outside.
More particularly attempts have been made to perfect a process and device for enabling very high production rates to be achieved with cycle lengths sufficiently short to be able to produce about 2 greens or more per minute.
Attempts have also been made to perfect a process and device which allows easy changing of the molds and also a device all the parts of which are easy to clean and maintain in service.